All your GIFVs display correctly on my Opera, what's actually being rendered is an MP4 and the biggest one weights 5 MB. If you click on the "download" link to download the legacy GIF, that one is of course much bigger and that's the size your rule refers to.

There you can see that the GIF of the failing GIFV is 26MB and the one that works in my comment is 23MB. But your Lights Fight video, which I can see perfectly, has a 66MB GIF file, so I think there must be something else that makes some of these fail.

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Vivaldi has the exact same issue, BTW. Chrome works fine.
Actually, Vivaldi 1.5 (stable), 32-bit on Win7-64 works fine with each of the OP's videos.

All those worked for me on Opera too. The ones that don't work in Opera don't work either on Vivaldi, at least in my case. For example, the one in my topic:http://i.imgur.com/z6a8tvH.gifv

I confirm the same result on my Win7 system for the Vivaldi 1.5 version for that MP4 link. It acts like the OP described: a momentary flash of the first frame, then it goes to a white screen with a link to the imgur forum post in the upper left corner. Alternatively: in Firefox 50, the first frame also appears on screen, but remains there; on the other hand, nothing else happens and the image remains static on-screen. Obviously, one's mileage can vary.

Hi everyone.
I have solved the issue, thanks to a completely unrelated problem that had the same solution. My OS is the N version of Win10, which means it is missing many features associated with Windows Media Player. I believe one of those features is necessary to play these files through Opera, though I don't know which. As soon as I downloaded the feature pack here